432 BASHFORD DEAN. 
the ventral lip of the blastopore is now separate from the yolk, 
but is closely apposed to it, as in the case of the dorsal lip, 
making the ceelenteron (c.) fissure-like; the inner germ layer 
is in contact with the yolk-cells near the point*; in the 
central region of the blastoderm it no longer exhibits the 
entoblast cells; their well-marked layer has apparently be- 
come merged with the yolk-cells.' 
In a still later gastrula (Pl. 32, fig. 33) the growth changes 
include :—the greater thickness of the lips of the blastopore, 
the retraction of the yolk-mass, the appearance of the cclen- 
teron as a distinct cavity (c.) and its extension forward under 
the dorsal lip (as far as the point *), the origin of the ectoblastic 
head mass, and of the middle germ layer. In the last regard 
this stage merits special attention: the mesoblast is found to 
arise peristomal (m.); on the dorsal side it arises from the un- 
differentiated tissue (of the tail mass), thence extends forward 
as a separate cell layer, and finally appears to be blended with 
the loosely associated cells of the entoblast ; ventrally the me- 
soderm (m.), although distinctly to be recognised, is not to be 
separated from the cellular elements of the entoderm. 
And finally, in the stage of the closing blastopore (Pl. 32, 
fig. 34), the embryo having surrounded about 180° of the egg’s 
circumference, the following characters appear:—The greatly 
increased size of the ectoblastic head mass (/.), and of the ceelen- 
teron (c.) together with the complete differentiation beneath 
the embryo ofthe middle germ layer (m.). The latter appears 
to have been differentiated in situ from the loosely associated 
cell mass shown in the preceding figure; it is separate 
anteriorly as far as the region immediately below the head 
terminal. The celenteron, now a deep cavity beneath the 
dorsal lip, extends forwards below the entire head; its hinder 
dilation (k.), immediately below the dorsal lip, is to be in- 
terpreted as representing Kupffer’s vesicle ;! beneath the ven- 
1 No critical attempt has ever been made to follow the actual mode of the 
growth of the embryo, as, for example, has been done by Morgan in the case of 
Ctenolabrus (1895, ‘ Journal of Morphology,’ vol. x, No. 2). 
2 Dean, op. cit., p. 42. 
